1. Field of the Invention
This invention relates to a self-lubricating seal assembly suitable for use in a Stirling engine.
2. Description of the Prior Art
As shown in FIG. 3, a Stirling engine has a cylinder 1 the interior of which is divided into an expansion chamber 3 and a compression chamber 4 by a working piston 2. The chambers 3 and 4 are interconnected via a heater, regenerator and cooler, none of which are shown, and the working space has an arrangement for sealing in a working gas such as helium. In order to minimize leakage of the working gas from one of the chambers 3, 4 to the other in a Stirling engine thus constructed, a gas seal assembly 6 is fitted into each of a plurality of ring grooves 5 provided in the outer peripheral surface of the working piston 2 and spaced apart longitudinally thereof.
In general, as illustrated in FIGS. 4 and 5, the gas seal assembly 6 fitted into the ring groove 5 of the working piston 2 comprises a stepped ring 7 and an expander ring 9. The stepped ring 7 comprises a composite material consisting mainly of a heat-curable resin such as polytetrafluoroethylene (PTFE resin) or polyimide resin filled with carbon fibers or carbon powder for the purpose of improving wear resistance and either thermal resistance or strength. The expander ring 9, which is made of steel, is placed in contact with the inner circumferential surface of the ring 7 to assure that the ring 7 will be held in pressured contact with the inner peripheral surface 8 of the cylinder. The stepped ring 7 has mutually opposing end faces 10, 11. The end face 10 is provided with a projection 10a extending circumferentially of the ring from its upper side, and the opposing end face 11 is provided with a projection 11a extending circumferentially of the ring from its lower side. The arrangement is such that the two projections 10a, 11a overlap each other. As a result, when the stepped ring 7 is installed in the groove 5, gaps A, B occur between the end face 10 and the projection 11a, and between the end face 11 and the projection 10a, respectively.
When a high-pressure gas from the expansion chamber 3 or compression chamber 4 acts upon the stepped ring 7, the latter is urged against a wall surface 12 of the ring groove 5, and working gas which has entered the bottom of the ring groove 5 cannot make its way to the gap B by virtue of the expander ring 9. An excellent sealing effect is thus obtained.
The opposing end faces of the ring 7 are not limited to the stepped configuration, for they can be straight-cut or angle-cut if desired.
In a Stirling engine, lubricating oil would mix in with the working gas and the oil would then attach itself to the cooler, regenerator and other components, thereby reducing the heat exchanging efficiency of the engine. In order to prevent this, the above-described seal ring assembly, which is self-lubricating (i.e., which does not employ a lubricant) is employed.
Recently, however, owing to the improved performance of Stirling engines and a worsening of the environment in which they are installed, there has been a significant increase in the ambient temperature within the engine and the conditions that result are quite severe insofar as a PTFE resin material is concerned. In addition, though there is a need for improved reliability in devices such as Stirling engines that use self-lubricating seal assemblies, a problem encountered is the large amount of wear that occurs when use is made of a dispersed material of carbon or glass fibers.